Methane is the second most powerful greenhouse gas whose concentrations are rising in the atmosphere but India recorded little growth in its emissions over the last few years. Methane is the second most powerful greenhouse gas after carbon dioxide and concentrations are rising in the atmosphere. Each tonne of methane emitted contributes roughly 25 times more to global warming than an equivalent amount of carbon dioxide. Because of its potency and quick decay in the atmosphere, countries have recognised that reduction of methane emissions can be an efficient means toward mitigating global warming. In India, methane is mostly emitted from agricultural sources (from cattle and other ruminating animals and from rice paddies) and its emissions are more uncertain than fossil fuel carbon dioxide because they are part of biological systems which are influenced by the environment. Methane emissions can vary dramatically from one place to another and over time, which makes them more difficult to account for in an inventory. India currently has the world's largest cattle population and is a leading producer of rice - because of this, methane accounts for approximately 20 per cent of India's total greenhouse gas emissions. India, like other Parties to the United Framework Convention on Climate Change, produces a report that accounts for their emissions of carbon dioxide, methane, nitrous oxide, and several synthetic greenhouse gases using underlying “activity data” (for example, counting the amount of a particular source, say vehicles, and scaling by a typical emission factor, the emission-per-vehicle). However, this inventory process, known as 'bottom-up' accounting, can have large uncertainties, particularly for gases like methane, and this is true in many parts of the world. The size of the uncertainty for many greenhouse gases (sometimes 100 per cent or larger) means the uncertainty can be as large as the emissions that are being reported. In light of new climate agreements such as the Paris Agreement, the implication of large uncertainties in national inventories is that it is difficult to both evaluate a baseline for reduction targets and to subsequently assess whether those measures have worked. The only way to better constrain national emissions is to use a variety of approaches, each providing its own independent estimate of emissions. It has been shown in other places in the world that an alternate methodology, known as 'top-down' emissions quantification, can be used to independently evaluate national emissions (although there are also uncertainties in the top-down approach) using measurements of methane concentrations in the atmosphere. Atmospheric measurements are key to evaluating and improving inventories - discrepancies between the two can highlight areas where work needs to be done to quantify emissions. The University of Bristol has studied emissions monitoring around the world. In our top-down study, we used measurements of methane concentrations from monitoring sites in India and from a satellite (monitoring methane concentrations around the world), to infer India's methane emissions from 2010-2015. We have concluded that there has been little change in India's methane emissions during this period and that the bottom-up reporting of these emissions by the country's authorities is accurate. This is in contrast to other countries, such as China, where atmospheric measurements have shown increasing emissions of the greenhouse gas in recent years or the United States, where large discrepancies have been found between reported emissions and those inferred from atmospheric observations. This finding should place some confidence in India's current methane inventory. We also found through observations of the atmosphere that methane emissions from India are enhanced each year between June and September - this is due to the rice production season. These enhanced concentrations over India can clearly be observed in the atmosphere. Atmospheric monitoring must become a standard component of emissions reporting for all countries. This independent assessment, in parallel with standard reporting, is essential for the success of international climate policies. By using a variety of approaches, we can reduce the uncertainties in our knowledge of greenhouse gas emissions. We now have the benefit of being able to measure some greenhouse gases from space, data which were used in our study, for example. Satellites allow us fill in gaps in places where it is difficult to have in-country monitoring. Satellites will play an important role in the future toward monitoring greenhouse gases and evaluating country and even city-scale emissions, however, they cannot be used completely independently of rigorous ground-based measurements. If all countries were to measure greenhouse gas measurements in their atmosphere and make them publicly available, then this would also help to add transparency to the process. Our findings are possible in part due to the atmospheric measurements made by India. It's encouraging to see that India's reporting of its methane emissions is fairly accurate and that the emissions are stable. To make climate agreements succeed, we need as much information as possible.
Dr Anita Ganesan is the NERC Research Fellow and Proleptic Lecturer at University of Bristol.